In this research, we propose to apply the twin method, a behavioral genetics technique, to problems in language acquisition. In particular, we will study three cases of delayed grammar learning in normal children: passive verbs, pronoun co-reference, and verbal inflection. The reason for the protracted learning of such constructions has been the source of a great deal of debate, with the conflict centered around the role of input. The twin method, by partitioning individual differences into relative genetic and environmental contributions, could shed light on this debate. In general, a large genetic contribution, or heritability, would implicate a maturational role in a construction, while a low contribution indicates greater importance of input and an intermediate level indicates a mixed explanation. This method also has the potential to enrich the study of language development in three ways. (1) It provides a novel way to group linguistic processes by providing relative heritability estimates, thereby suggesting common sources for some and not others. (2) It provides absolute estimates of heritability, which could tell us to what extent delays in normal children are predictable from the delays demonstrated by other children in their family. In addition, it may tell us to what extent intervention could be effective in speeding up a child s language development. (3) finding heritability in language development would corroborate claims that language has some innate basis and support theories that language evolved by natural selection.